1. Field of the Invention
The present invention relates to a communication system using asynchronous ,transmission mode (ATM) cells for use in a broadband ISDN or the like, and in particular, to a police system of ATM exchange for preventing speeds of ATM cells which are received from subscribers from exceeding a predetermined band.
2. Description of the Related Art
FIG. 1 shows an example of the construction of a broadband ISDN system. In the figure, a user terminal 1 of a transmission side of a broadband ISDN system divides digitized information such as sound, data, image, and so forth into ATM cells of fixed length. At the beginning of each ATM cell, a header which represents the address of the cell is added. The resultant ATM cell is sent over a transmission line 3 through an ATM exchange 2. On a receive side of the system, an ATM exchange 4 exchanges the ATM cell and sends it to a user terminal 5 of the address.
In an asynchronous transmission mode (ATM) system which is a broadband ISDN transmission system, the transmission speed of subscriber lines is designated as 155.52 Mbit/S or the quadruple value thereof. Data to be transmitted is divided into blocks of fixed length. These blocks are referred to as ATM cells.
FIG. 2 shows an example of the format of an ATM cell. In the figure, the total length of each ATM cell 6 is 53 bytes. The ATM cell 6 is composed of a header field 7 of five bytes and an information field 8 of 48 bytes. The header field 7 stores a virtual path identifier (VPI), a virtual channel identifier (VCI), and so forth. The VPI and the VCI are used to identify each subscriber. The information field 8 stores transmission data. The last one byte of the header field 7 is a header error control (HEC) in which header correction control data is stored.
FIG. 3 shows the construction of the header portion of the ATM cell of FIG. 2 in detail. In the figure, the virtual path identifier (VPI) is stored in the first and second bytes of the header portion of five bytes and the virtual channel identifier (VCI) in the second to fourth bytes thereof. The designating method of the VPI and VCI for each subscriber (terminal) has been set forth in CCITT Recommendation. However, actually, any values can be designated to the VPI and VCI.
As described above, the present invention relates to a police system of ATM exchange. The police system serves to manage the band of cells which are transmitted from the user terminal 1 to the ATM exchange 2 shown in FIG. 1. The police system verifies whether or not the band of cells received from each subscriber deviates from a band which has been arranged between the exchange side and the subscriber side and if necessary to stop receiving cells from the subscriber so as to effectively use the band resources of the exchange and network and prevent the network from becoming congested.
Since the ATM exchanging technology is a new art, there are not so many police systems which have been disclosed. As an example of this art, a technique for counting the number of cells which are transmitted in a particular unit time is known. However, in this technique, since band congestion could not be detected, it was impossible to prevent excessive cells from entering a network. Thus, the network could not be protected from band congestion which took place instantaneously.